Valves of the type under discussion are described in U.S. Pat. Nos. 4,200,116 and 4,291,721 as well as in Ser. No. 341,829 filed Jan. 22, 1982 and the art of record in these cases.
It is imperative to equip an apparatus or circuit containing a fluid under pressure, for instance a boiler, with a safety valve adapted to vent the volume which is to be protected into the atmosphere when the fluid pressure attains a predetermined maximum value beyond which a pressure increase might be dangerous to equipment or personnel or to the environment.
Boilers of the type which must be protected with such safety valves include industrial, ship and power plant boilers and the fluid pressure vessels of nuclear reactors and the like.
A pilot-controlled valve generally comprises a valve body mounted along a passage connecting the vessel with the ambient atmosphere, a valve member mounted in this body for movement toward and away from a valve seat and a piston-and-cylinder arrangement generally surmounting this body and having a piston connected to the valve member and responding to a pilot pressure for biasing the valve member into its closed position at vessel pressures below the predetermined level while enabling displacement of the valve member into an open position when the vessel pressure reaches the predetermined level to relieve the pressure in the vessel by venting the latter to the atmosphere.
Because of the need of a piston-and-cylinder arrangement in addition to the valve body and valve member, this system is relatively complex and the complexity is enhanced by the need for thermal barriers, sliding seals and like elements which are required because of the actuating system provided for the valve member.
The reliability of the valve often is cast into doubt because of the frictional resistance to movement of the valve member and the need for considerable pressure when the valve member is in its normal position, i.e. is closed.
The problem of venting the cylinder also arises in this system which may not be as compact as is desired because of the additional structure which must be mounted upon the valve housing.
While a pilot-controlled safety valve is known which utilizes an internal cylinder and which is far more compact than the external cylinder valve previously described, most of the other disadvantages described above remain, especially when the device is used at high temperatures.